Symptom:
Adding a custom "New ITem Form" to a page in SharePoint Designer 2010. The new form adds properly and configures normally, but when you try to use it to submit a new item, you get this error:
Failed to get value of the "Attachments" column from the "Attachments" field type control. See details in log. Exception message: Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)..
Solution:
Follow best practices. :)
This was in our dev environment and I was hiding the out of the box new item form and adding a new (custom) one to the page. The error goes away when you create a new form and stop fiddling with the out of the box pages. Harumpf.